Support Binsparse Format

[willow-ahrens]

Hi there! Thanks for maintaining such a great library! I'm wondering, would it be possible to add support for the Binsparse file format? I think there's some discussion of adding support to the C library as well, so perhaps this should wait on that?

[rayegun]

I think I can prototype before it's ready in C.

I'll give it a go. I'm almost done with a full rewrite of this to prepare for some other stuff like Finch extension.